This invention relates to a liquid crystal display device suitable for use in a color display such as a color television set.
A liquid crystal color display device has the following construction. A plurality of transparent electrodes are formed on each of two transparent substrates and a color filter is provided over the transparent electrodes on either of the two transparent substrates. The two transparent substrates are faced to each other with the electrodes-formed sides inside. An orientational film is formed for orientation, covering the entire surface of that side of each transparent substrate facing the other transparent substrate. TN (Twisted Nematic) liquid crystal is filled in the space between the orientational films. A deflecting polarizing plate is provided on the other side of each transparent substrate. In the liquid crystal display device of the above construction, when voltage is applied to transparent electrodes, the optical characteristic of the liquid crystal at the positions where voltage is applied will change. Specifically, the liquid crystal transmits light or shields it depending upon the combination of the deflecting axis directions of the two deflection plates. Thus, a color image is presented on the display device as voltage is applied selectively to transparent electrodes.
To prevent deviation by parallax, the color filter is normally positioned as close to the liquid crystal layer as possible. Liquid crystal color display devices are divided into two types according to the position of the color filter: the one with a color filter provided over the transparent electrodes formed on a transparent substrate, and the one with a color filter provided on a transparent substrate and with transparent electrodes formed over the color filter.
In a typical liquid crystal display device, a plurality of X-electrodes are formed on one of two transparent substrates and a plurality of Y-electrodes on the other transparent substrate, the X-electrodes and Y-electrodes being orthogonally arranged and being made to face each other to form display picture elements at the intersections thereof. The liquid crystal display device of this type is driven in a so-called time division drive method, that is, by scanning the X-electrodes sequentially and applying display or non-display voltage to the Y-electrodes corresponding to the picture elements on the selected X-electrodes. Liquid crystal materials are relatively slow in response, and have a property that the light transmittance becomes constant after being scanned repeatedly. For a liquid crystal display device designed for high duty cycle operation and expected to provide a wider visual angle and a higher contrast, however, it is desirable that the light transmittance of the liquid crystal change sharply when applied voltage reaches a specified threshold value.
In the liquid crystal color display device with the color filter formed over the transparent electrodes, applied voltage is reduced by the color filter, causing slower change in the light transmittance around the threshold value of applied voltage. As a result, a clear image cannot be obtained on the display device. For this reason, the liquid crystal color display device with the transparent electrodes formed over the color filter is more preferable. For the display device of this type, however, because of the thermal resistance limitation of the color filter, it is not possible to raise the substrate temperature high enough to form transparent electrodes of a small resistance which is essential to a liquid crystal display device of a high duty ratio. To solve this problem, a liquid crystal display device has been proposed in which the resistance of the transparent electrodes is lowered by forming metal electrodes on the transparent electrodes.
FIG. 5 is a plan view of a transparent substrate 4 with a color filter 1, transparent electrodes 2 and metal electrodes 3 formed thereon, for use in a typical liquid crystal display device by the conventional art. FIG. 18 is a sectional view of the transparent substrate 4 cut along the line VI--VI of FIG. 5. In FIG. 5, transparent electrodes 5 formed on another transparent substrate to be arranged as opposed to the transparent substrate 4 are shown by broken lines. Referring to FIG. 18, the color filter 1 is formed into a pattern of, say, a red color filter 1R, a green color filter 1G, a blue color filter 1B, a red color filter 1R, and so on. In FIG. 5, therefore, the transparent electrodes 2 distinguished by vertical lines look red, those distinguished by oblique lines look green, and those distinguished by horizontal lines look blue.
Thus, according to the conventional art, the electrode for applying voltage to the liquid crystal layer is made up of the combination of a transparent electrode 2 and a metal electrode 3. The portion of the transparent electrode 2 on which the metal electrode 3 is placed shields light, resulting in reduced aperture of the picture elements realized by the combination between the transparent electrodes 2 and transparent electrodes 5. This hampers presentation of a clear picture.